D2D communication is a technology which allows a user equipment (UE) to directly communicate with an adjacent UE over licensed or unlicensed frequency bands under controls of a wireless communication system, and such technology would enable a wireless communication network to increase system spectral efficiency, to reduce transmitted powers of each terminal, and to alleviate the resource consumptions of the wireless communication network. For both commercial and public safety purposes, one of the design considerations of D2D communication could be to make D2D communication available for a UE regardless of whether the UE is within the coverage of a cellular network, partially within the coverage of a cellular network, in between the coverages of two cellular networks, and even not at all within the coverage of a cellular network. Also, potentially a very large quantity of concurrently participating UEs may need to be considered. In order to satisfy the aforementioned design considerations, an N-hop synchronous network could be utilized.
FIG. 1A illustrates network coverage without using an N-hop synchronous network. Since a base station 101 or a cluster head has a limited coverage range 102, the base station 101 or cluster head may not be able to reach a UE 103 that is situated outside of its coverage range 102. FIG. 1B illustrates a concept of an N-hop synchronous network. One of the ideas behind the N-hop synchronous network is that since it might be difficult to have one SYN source covering all UEs situated outside the network coverage, some UEs 111 could take on the task of forwarding or even independently providing time alignment and frequency synchronization information to cover UEs outside of the coverage range 102.
Before D2D communication can commence between two or more UEs, timing alignments and/or synchronizations of the UEs to a network would need to be accomplished. A UE would be able to synchronize to a network directly or indirectly by synchronizing to a D2D Synchronization Signal (D2DSS) from which timing and synchronization information could be obtained. FIG. 1C illustrates a resource pool for D2D communication. After receiving a D2DSS, a UE that is within a network coverage or outside of a network coverage would be aware of a resource pool in terms of a specific time slot and/or a frequency spectrum which may contain resources for transmitting D2D data.
FIG. 1D illustrates a hypothetical N-hop synchronous network. Such hypothetical N-hop synchronous network may include but not limited to a cluster head 141, at least one D2D Synchronization Source (SYN source) 142, and at least one normal UE with D2D capability 143. A cluster head 141 could be a base station (e.g., eNB) or a UE.
A UE 143 may synchronize to a network by receiving a D2D Synchronization Signal (D2DSS) from which a UE could be able to accomplish timing alignment and frequency synchronization. Such D2DSS could be provided by a cluster head 141 which could be a base station (e.g., eNB) or a UE. If the cluster head 141 is a base station, the D2DSS would be a primary synchronization signal (PSS) and a secondary synchronization signal (SSS) which are transmitted by the base station. Under the circumstance in which a UE cannot receive timing alignment and frequency synchronization information from a network, the UE may act as an independent cluster head 141. For the purposes of providing network coverage outside of the network coverage or near the boundary of the network coverage, a D2DSS could be transmitted or relayed by a SYN source 142 which could be another UE with D2D capability.
A SYN source 142 would scan for a first D2DSS as a reference from the cluster or from other SYN sources 142. If a first D2DSS has been detected and subsequently selected as a reference, the SYN source 142 may synchronize to the first D2DSS before it may transmit a second D2DSS which would be derived based on the first D2DSS. If no D2DSS has been detected at all, the UE may nevertheless transmit the second D2DSS without timing reference from the first D2DSS. Any SYN source 142 may re-select the D2DSS as a reference if the SYN source 142 has detected a change in the D2DSS.
In order to synchronize a large quantity of UEs situated within or outside of the network, one method could be to allow all UEs to be SYN sources. However, such method may bring about unsatisfactory consequences such as unnecessary energy consumption and D2DSS contamination. Assuming that there is no timing alignment information available for D2DSS forwarding timing, the timing difference between SYN sources because of propagation delay may result in D2DSS contamination. Therefore, the number of SYN sources would need to be reduced.
To reduce the quantity of UEs being SYN sources while keeping the large synchronization areas, a mechanism that involves using “predefined Reference Signal Receiving Power (RSRP) Threshold” could be used to reduce the number of SYN sources in conjunction with a mechanism that involves using “Cluster Head Muting” to reduce the number of cluster heads.
For “predefined Reference Signal Receiving Power (RSRP) Threshold,” if a UE cannot detect other cluster heads or SYN sources, the UE would become cluster head which has hop count=0. The hop count is the number of hops from the cluster head to the SYN source. A UE would become a SYN source only if the maximum received power from neighboring SYN sources does not exceed a predefined threshold (e.g., −80 dBm/−103 dB path gain). In general a UE would become a SYN source with hop count N if the UE detects or synchronizes to a SYN source with lower hop count (N−1). For “Cluster Head Muting,” when a SYN source detects two or more cluster heads, the SYN source would select one of them as synchronization source. This means that the remaining cluster heads not selected by the SYN source may mute their operations when receiving a D2DSS from the SYN source with hop count such as n=1.
FIG. 2A˜2D illustrates various hypothetical scenarios involving N-hop synchronous network operating under the aforementioned first mechanism and second mechanism. The hypothetical N-hop synchronous network of FIG. 2A would include a cluster head 201 having a first power range 202 and a second power range 203. UEs (e.g., 204) within the first power range 202 would synchronize to the cluster head 201 and the maximum received power from the cluster head 201 exceeds a predefined threshold. UEs (e.g., 205) between the first power range 202 and the second power range 203 would also synchronize to the cluster head 201 and the maximum received power from the cluster head 201 does not exceed a predefined threshold. Thus these UEs (e.g., 205) would serve as a SYN source. For UEs (e.g., 206) that cannot detect other cluster heads or SYN sources, these UEs (e.g., 206) may become cluster head which has hop count=0.
Similarly, for the hypothetical N-hop network of FIG. 2B, the UE 211, it would serve as a SYN resource as the maximum detected power from the cluster head 210 is below a predefined threshold. In the hypothetical N-hop synchronous network of FIG. 2C, a situation of “Cluster Head Muting” could be leveraged, when a UE 212 serving as a SYN source detects two cluster heads 213 214. In this situation, the UE 212 would select one of the two cluster heads 213 214 as a source to synchronize itself to. Assuming that the cluster head 213 is selected, the cluster head 214 that has not been selected by the UE 212 may mute its operation when receiving a D2DSS from the UE 212 with hop count such as n=1.
However, this mechanism that involves “predefined Reference Signal Receiving Power (RSRP) Threshold” and “Cluster Head Muting” would encounter difficulties as illustrated in the hypothetical N-hop synchronous network of FIG. 2D. In such extreme case, this mechanism may cause substantial time differences due to the skewed arrangement of the cluster head and SYN sources. If a SYN source bases upon a D2DSS previously received to forward a subsequent D2DSS (i.e., no timing advance is applied for D2DSS forwarding timing), the subsequent D2DSS timing could be propagated with a delay.
For FIG. 2D, assuming that cluster head 220 initiates a D2DSS which has a propagation delay of T1_0 which could be zero, the SYN source 221 would receive the D2DSS with a propagation delay of T1_1, the SYN source 222 would receive the forwarded D2DSS with a propagation delay of T1_2, and the SYN source 223 would receive the forwarded D2DSS with a propagation delay of T1_3. This would mean that if the UE1 241 engages in D2D communication with UE2 242 as in step S250, the messages would normally be not synchronized and thus a Cyclic Prefix (CP) is needed to cover the time difference between, for example, the D2D transmitter of UE2 242 and the D2D receiver of UE1 241. In a general N-hop Synchronous Network, the length of CP shall be larger than TA+N×(TA/2), wherein N is maximum hop count of the N-hop Synchronous Network and TA is the maximum propagation delay within a D2DSS coverage. This means that, the length of CP for the scenario of FIG. 2D would substantially exceed the current CP of Long Term Evolution (LTE) and LTE-advanced (LTE-A) communication system. Therefore, a different solution could be proposed to at least avoid the aforementioned problem.